I notice that the introductory email contains the following words: "spambot", "bot", "spammer" etc.

Maybe rephrasing it without including these would make the content a bit less sensitive for Google?
examples:
"to prove that you are not a spammer", use "to prove that you are a real human".
"bot/spambot" -> "automated fraud attempts"

Maybe this is not what is triggering the classification as spam but it could be worth trying.

Can you try sending the original email + another "rephrased" version at my firstname.lastname@gmail.com address? I will see if these are categorized as spam.